1. Field of the Invention
The present invention relates to a developing device and an image forming apparatus.
2. Description of Related Art
Conventionally, one-component development methods and two-component development methods are employed in electrophotographic developing devices. In one-component development methods, toner makes contact with each member of a developing device at a part for supplying toner, a part for charging toner, a part for neutralizing toner, and a part for collecting toner, and as a result stress is applied to the toner. Thermoplastic resins are used to form a toner, and inorganic fine particles serving as a flowability modifier are attached on the surface of the toner. Therefore, due to the above-mentioned stress, thermal variation may be caused and the inorganic fine particles may be buried in the toner surface. In the case of high-speed apparatuses, the stress applied to toner is even greater since the rotational speed of each component thereof is high. For this reason, the speed is limited. In addition, in recent years, the particle size of toner is remarkably reduced as the image quality is enhanced, and a toner having the diameter of 6 [μm] or less is not uncommon. Since such a small-sized toner is treated with many post-treatment materials and its fluidity is degraded, the aggregation of the toner and burying of the post-treatment materials due to the above-mentioned stress frequently occur. In addition, there has been a growing trend toward low-temperature fixation in view of environmental considerations. This trend results in decreased thermotolerance, which is disadvantageous in terms of the above-mentioned stress.
In two-component development methods, toner charged by triboelectric charging of the toner and carrier is attached to an electrostatic latent image formed on an image bearing member to thereby develop the image. In a developing device, the ratio of the toner and carrier is kept at a certain ratio to maintain the charging state of the toner. However, when a toner image is developed on the electrostatic latent image formed on the image bearing member, the toner becomes insufficient. Therefore, toner is supplied by replenishing means. The toner supplied by the replenishing means is uncharged toner, which is charged when it is stirred with a developer in a developing device by stirring means in the developing device, and conveyed by conveying means in the developing device. Since in such a method charging is performed by mixing particles, the stress applied to toner is small. Accordingly, in comparison with the case of one-component development methods, the toner has a longer life, and besides, has greater adaptability to high speed. Since the surface area of the carrier is greater than that of the toner, the carrier is less likely to be contaminated by the toner adhered to the surface of the carrier. However, after a long period of use, contamination (spent) on the surface of the carrier increases, and thus the performance for charging toner decreases with time. As a result, problems such as fogging and toner scattering occur. In order to extend the life of two-component developing devices, it is conceivable to increase the amount of carrier housed in the developing device. In that case, however, the size of the developing device is increased, which is undesirable.
To solve the above-mentioned problems of two-component developers, Japanese Examined Patent Publication No. 2-21591 discloses a developing device of a trickle type in which a developer is supplied to the developing device little by little, and a developer whose charging performance is degraded is discharged from the developing device little by little, to thereby suppress the increase of degraded carrier. This developing device utilizes change in volume of the developer to discharge the excess and degraded developer so as to maintain a substantially constant volume level of the developer in the developing device. With this developing device of the trickle type, the degraded carrier in the developing device is replaced by newly supplied carrier, and the charging performance of the carrier in the developing device can be maintained at a substantially constant level.
A device of a trickle type has been conceived in which, for the purpose of simplifying the configuration of the device, toner and carrier are supplied from a developer cartridge that houses a developer obtained by mixing the toner and carrier at a constant ratio so as to simultaneously supply the toner and carrier. However, a toner consumption amount per one recording sheet (hereinafter referred to simply as “toner consumption amount”) differs greatly depending on the image to be formed. For example, a document with only letters tends to consume a small amount of toner while a photographic image tends to consume a large amount of toner. On the other hand, degradation of carrier depends on the number of stirring carried out in the developing device, in other words, the number of image formation (the number of recording sheets), not on the toner consumption amount.
Therefore, when the developer obtained by mixing the toner and carrier at a constant ratio is supplied in accordance with the toner consumption amount, then a large amount of toner has to be supplied in the case where the toner consumption amount is large, and as a result, carrier is unnecessarily supplied and wastefully discarded. Conversely, in the case where the toner consumption amount is small, it is not necessary to supply a large amount of toner, and therefore, the amount of carrier to be supplied is decreased, and, so to speak, the metabolism of the carrier is deteriorated, which may degrade the charging performance. For this reason, techniques are proposed in which toner and carrier are not previously mixed, but are separately supplied (see, for example, Japanese Patent Application Laid-Open No. 2001-183893, and Japanese Patent No. 2986001).
In the technique according to Japanese Patent Application Laid-Open No. 2001-183893, a toner concentration detector that stably detects the toner concentration (the ratio of toner in the developer) in a steady state in a developing device is provided at a position where supplied toner and carrier are sufficiently mixed with a developer in the developing device. When the toner concentration detected by the toner concentration detector is lower than a previously set toner concentration, toner replenishment is performed for the toner consumed by the development. When the toner concentration detected by the toner concentration detector is not reset to a predetermined value, or is further decreased after the toner replenishment, a warning message is displayed on a monitor, for example. In this manner, a user can recognize that the toner replenishment has not been properly performed, and can recognize the necessity of measures such as reconditioning of a toner replenishment mechanism and replacement of a toner bottle that is provided above a toner hopper and configured to supply toner to the toner hopper.
On the other hand, when an error occurs in a carrier replenishment mechanism or when the carrier hopper is empty, the carrier replenishment is not properly performed. Since carrier in the developing device is not typically consumed, however, the toner concentration detected by the toner concentration detector does not change, and thus the user cannot recognize the fact that the carrier replenishment has not been properly performed. In an early stage, a carrier replenishment failure has no influence on formed images and raises no problem. However, since degraded carrier is not replaced with newly supplied carrier, degradation of carrier is gradually facilitated, and accordingly the toner charging performance is degraded with time. As a result, the problems such as fogging and toner scattering occur.
It is to be noted that, while Japanese Patent No. 2986001 discloses a technique in which the toner concentration in a developing device and the amount of discharged developer are detected, whether the carrier replenishment has been properly performed cannot be recognized since the developer to be discharged is strongly influenced by factors (such as inclination of the developing device and change in volume of the developer due to environmental changes) other than the carrier replenishment.